Diagnostic device for gas turbine ignition system

ABSTRACT

A device for diagnosing the state of health of an ignition system is provided, where the system includes at least one spark producing channel comprising an exciter, output circuit and igniter plug. The device provides a diagnosis of the state of health for both the exciter and igniter plug by monitoring the high energy pulses at the output of the exciter. By monitoring the ignition system at an intermediate point in the system such as the output of the exciter, the sensor and electronics of the device may be completely contained within the electronic environment of the exciter, thereby avoiding any need for attaching sensors at the output of the system adjacent to the igniter plug in order to diagnose the plug&#39;s state of health. As an alternative to the device being built into the ignition system, it can be incorporated into automatic test equipment that produces high energy pulses for delivery to an igniter plug to be tested. The device is capable of diagnosing failure of either the exciter or the igniter plug and may also be configured to detect the impending failure of the plug.

This is a continuation of copending application Ser. No. 07/557,973,filed on Jul. 26, 1990, now U.S. Pat. No. 5,155,437.

TECHNICAL FIELD

The invention generally relates to ignition systems for gas turbineengines and, more particularly, to monitoring and diagnostic devices forsuch systems.

BACKGROUND OF THE INVENTION

The monitoring of ignition systems for gas turbine engines is ofparticular interest because such systems are of critical importance tothe safe operation of aircraft incorporating these types of engines. Bymonitoring the performance of ignition systems in gas turbine engines,an indication that the system is malfunctioning can be obtained. Byproviding an indication of a malfunctioning ignition system, a measureof safety is obtained that can be of particular importance in ensuringthe ignition system is capable of restarting an engine after a flameouthas occurred, or to initiate a maintenance cycle prior to the nextflight.

In monitoring ignition systems of gas turbine engines, the state ofhealth of the igniter plug has in the past been the focus since theigniter plug is the component of the ignition system with the shortestaverage useful life. Failure or malfunctioning of other components ofthe ignition system, however, may occur and the typical monitoringsystem fails to identify failures or malfunctioning of these othercomponents. Indeed, some monitoring devices may actually falselyindicate a properly operating ignition system when the system is in factmalfunctioning or failing, others may indicate a failure when noneexists causing unnecessary maintenance.

An ignition sequence is typically initiated by a narrow high voltagepulse generated by an exciter circuit. For a successful ignition, thehigh voltage pulse is discharged at the igniter plug, thereby generatinga spark. There have been attempts to analyze the voltage pulse from theexciter and the following voltage waveform generated by the spark inorder to diagnose the health of an ignition system. In the past,however, such monitoring systems could only provide an indication of thehealth of the igniter plug and failed to monitor or diagnose the stateof health of other components of the ignition system that may lead tofailure of the igniter plug.

For example, U.S. Pat. No. 4,760,341 to Skerritt discloses a monitoringdevice that senses the electric field generated by a signal at the inputto the igniter plug of an ignition system. The monitoring devicereceives the signal generated by the electric field and detects if theinput signal to the plug is maintained longer than a predetermined timeperiod and above a predetermined voltage level. If the input signal ismaintained longer than the predetermined time period, the deviceindicates the plug is deteriorating. If the voltage of the input signalfails to reach the predetermined level, however, the monitor of theSkerritt patent also interprets this failure as a deteriorating plugwhen in fact the exciter may be degraded and the igniter is functioningproperly.

In addition to measuring the width of the high voltage pulse, themonitoring device in the Skerritt patent also measures the energydischarged through the plug during the spark event. If the total energydelivered in the spark event signal is satisfactory and the signal tothe plug is not too long, the monitoring device provides a pulse output,indicating that the igniter plug is operating properly.

Although measurement of the total energy delivered to the plug inresponse to the high energy pulse is a useful complement to themeasurement of pulse duration, the two measurements fail to provide theuser with anything other than a general indication that the ignitionsystem is malfunctioning. More specifically, the measurements of theSkerritt monitoring device do not distinguish between a failing devicefor discharging the high energy pulse (i.e., the igniter plug) and afailing device for generating the high energy pulse (i.e., the exciter).

SUMMARY OF THE INVENTION

It is the primary object of the invention to monitor the health of eachof the igniter plug and the exciter circuit in an ignition system bydetecting abnormal conditions in the waveforms of the system associatedwith a spark event.

It is also an important object of the invention to detect malfunctioningof the igniter plug at a location within the ignition system that isisolated from the extreme environment of the igniter plug and isimplemented without the need of an expensive sensor coupling to theigniter plug or its input leads; but rather by sensing perturbations inthe waveforms of existing signals. In this connection, it is also anobject of the invention to monitor the state of health of the igniterplug at a location within the ignition system that is remote from theplug itself so that the monitoring device can be effectivelyincorporated into the same apparatus as the exciter circuitry andtotally isolated from the extreme environment of the igniter plug.

It is a related object of the invention that no additional wires orconnectors are required on the downstream side of the exciter in orderto accomplish the diagnostic monitoring.

It is yet another important object of the invention to diagnose thestate of health of the ignition system by distinguishing between thefailure of the plug and the failure of the exciter circuit. In thisregard, it is a related object of the invention to prevent falsediagnosis of the state of health of the igniter by requiring the exciterto be diagnosed as healthy before the igniter is diagnosed as failed.

It is another important object of the invention to accurately monitorthe health of both the igniter plug and the exciter circuit for anignition system by distinguishing between actual failure of the plug orexciter circuit and the occasional irregularities in the high energypulse that may occur as a result of normal operation in the severeenvironment of a turbine engine.

It is another important object of the invention to provide an indicationof the impending failure of an igniter plug in an ignition system so asto provide an opportunity for initiating preventive maintenance.

It is still another object of the invention to minimize the number ofleads required to communicate to a remote location the diagnosticinformation derived from the monitoring of the ignition plug and excitercircuit.

It is another object of the invention to provide a monitoring system foran ignition system that is easily adapted as a transportable automatictest equipment (ATE) separate from the turbine engine and its ignitionsystem.

Other objects and advantages will become apparent with reference to thefollowing detailed description when taken in conjunction with thedrawings.

The invention achieves the foregoing objects by providing a monitoringdevice for an ignition system of a gas turbine engine that comprises anexciter detector and an igniter detector, each monitoringcharacteristics of high energy pulses delivered from an exciter to anoutput circuit of the ignition system for generating a spark at anigniter plug. In response to the monitoring of the high energy pulses,the exciter detector provides an indication of the exciter's state ofhealth and the igniter detector provides an indication of the igniterplug's state of health.

In the exciter detector, the voltage levels of the high energy pulsesare monitored to determine whether they are persistently less than apredetermined value representing a nominal minimum voltage generated bythe exciter when healthy. In the igniter detector, the rate of dischargefor the high energy pulses from the exciter into an output circuit ofthe ignition system is monitored to determine whether the rate ofdischarge is persistently less than a predetermined rate representing anominal minimum rate of discharge for a healthy igniter plug.

In order for the monitoring device to sense the high energy signals fromthe exciter, a high impedance voltage divider network connects theoutput of the exciter to ground. A signal from a node of the voltagedivider provides the input signal for each of the exciter and igniterdetectors. For purposes of economy, a safety resistor, typically presentat the output of the exciter, may be incorporated into the voltagedivider.

The exciter and igniter detectors are responsive to the high energypulses generated by the exciter and the discharging of the high energypulses into the output circuit. In a properly operating ignition system,a healthy exciter produces high energy pulses of at least apredetermined minimum voltage. For a healthy igniter plug, the outputcircuit stores the high energy pulse into an inductor and quicklydischarges the energy of the pulse as a spark at the igniter plug. Ifthe igniter plug fails to spark, the input to the output circuit appearsas a virtual open circuit to the high energy pulses and the pulsesdischarge through the voltage divider network. The rate of discharge forthe high energy pulses through the voltage divider network is much lessthan the rate of discharge for the pulses through the igniter plug.Therefore, the ignition detector monitors the rate of discharge of thehigh energy pulses and determines from that rate the igniter plugs stateof health.

To prevent the false indication of a failed igniter plug as a result ofthe upstream failure of the exciter, the response times of the exciterand igniter detectors are such that the exciter detector response issignificantly faster than the igniter detector. A diagnostic outputcircuit responsive to the exciter and igniter detector provides the userof the monitoring device with an indication of the state of health ofthe ignition system and ensures against a false indication of a failedigniter plug by ignoring an indication of a failed igniter plug if theexciter detector is also providing an indication of a failed exciter.

The monitoring device of the invention may be used to monitor a singlechannel of an ignition system--i.e., an exciter and associated outputcircuit and igniter plug. Alternatively, the monitoring device may beused in connection with an ignition system comprising multiple channels.For a multiple channel ignition system, the monitoring device includesexciter and igniter detectors for each channel. In order to minimize thecabling necessary to communicate the diagnostic signals of themonitoring device to a display in a multi-channel system, the signalsfrom the various exciter and igniter detectors are first encoded andthen communicated to a remote display via a thin cable. At the remotedisplay, the signals are decoded and the state of health of the ignitionsystem is indicated to a user.

In an alternative embodiment, the diagnostic system of the invention maybe placed in a stand alone automatic test equipment (ATE) environment sothat it can be incorporated into a structured maintenance routine. TheATE may include an exciter and output circuit in order to deliver thesame type of high energy pulses provided an igniter plug by the ignitionsystem. A technician or other maintenance personnel disconnects theigniter plug from the ignition system either by physically removing theplug or disconnecting the plug at its cable connection to the outputcircuit of the ignition system. Once disconnected, the igniter plug canbe connected to receive high energy pulses from the exciter and outputcircuit associated with the ATE apparatus and the monitoring device willreport to the maintenance personnel the state of health of the igniterplug.

In addition to detecting persistent failure of the exciter and igniterplug of a channel, the monitoring device may also detect theintermittent failure of either the exciter or igniter plug and report adiagnosis and response thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the monitor of the invention in itsintended environment of an ignition system in a turbine engine;

FIG. 1A is a schematic diagram of an exemplary output circuit for theignition system of FIG. 1;

FIGS. 2A-2B are exemplary voltage waveforms for a high energy pulsedelivered to an igniter plug by an exciter circuit in the ignitionsystem of FIG. 1, where FIG. 2A illustrates an exemplary waveformassociated with a healthy igniter and FIG. 2B illustrates an exemplarywaveform associated with a failed igniter;

FIG. 3 is a schematic diagram of the monitor of FIG. 1 implemented in ananalog manner and in accordance with a preferred embodiment; and

FIG. 4 is a schematic block diagram of a digital circuit for monitoringthe state of health of the igniter plug in accordance with the inventionsuch that both actual failure and impending failure of the plug may bediagnosed.

While the invention will be described in some detail with reference to apreferred embodiment and an alternative embodiment illustrated in thedrawings, it is to be understood that this description is not intendedto limit the scope of the invention. On the contrary, applicant intendsthe scope of the invention to cover all alternatives, modifications andequivalents that fall within the spirit and scope of the appendedclaims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning to the drawings and referring first to FIG. 1, a monitoringdevice 11 according to the invention is responsive to a series ofvoltage signals derived from a corresponding series of high energypulses provided at the output of each of a pair of exciters 13 and 15 ofan ignition system 17 for use by a pair of output circuits 19 and 21 andigniter plugs 1 and 2 to generate sparks. Each one of the pairs iscommonly referred to as a channel. Although the ignition system 17 asillustrated in FIG. 1 includes two channels, it will be appreciated thatthe ignition system may include only one channel or alternatively aplurality of channels.

As is well known in the art, each of the exciters 13 and 15 and outputcircuits 19 and 21 is typically located externally from the turbineengine 23, whereas each of the igniter plugs 1 and 2 is of courselocated within a combustion chamber of a turbine engine 23 as suggestedby FIG. 1. A fuel source 25 provides fuel to the combustion chamberwithin the engine 23 where it is ignited by sparks generated by each ofthe igniter plugs 1 and 2. The two channels of an exciter, outputcircuit and igniter plug is a typical installation in a turbine engineused in aircraft since it provides a redundancy that protects againstfailure of the ignition system in flight. Because the two channels of anexciter, output circuit and igniter plug are redundant, those skilled inthe art of ignition systems for gas turbine engines will appreciate thatthe following description with reference to one channel of the exciter13, output circuit 19 and plug 1 applies equally well to the otherchannel of exciter 15 and igniter circuit 21.

Each of the output circuits 19 and 21 comprises a unipolarity diode D1,an output inductor L1 and a high voltage output connector 54 asillustrated in FIG. 1A. The output connector 54 is a conventional highvoltage coupling that is useful in connection with an alternativeapplication of the invention. Specifically, a monitoring device like themonitoring device 11 of FIG. 1 may be incorporated into automatic testequipment (ATE) that reproduces the function of the ignition system 17so as to provide high energy ignition pulses to either of the igniterplugs 1 or 2. The state of health of each igniter plug 1 and 2 may bechecked by a technician at any time merely by disconnecting the plugfrom the ignition system and connecting it to an output of the ATE thatprovides calibrated high energy pulses. The monitoring device internalto the ATE functions the same as the monitoring device 11 and providesthe technician with a means to quickly check the ignition system toensure it is functioning properly. Although not as convenient asdisconnecting the igniter plug at the high voltage connector, the ATEapparatus may alternatively incorporate a socket for receiving igniterplugs such that the technician physically removes the plug from theignition system and places it in the ATE for testing its state ofhealth.

As can be appreciate from FIG. 1A, each of the output circuits 19 and 21provides for unipolar discharging of the high energy pulses from theexciters 13 and 15. The invention may also be used in connection with abipolar discharge, however, and in this regard the unipolarconfiguration of FIG. 1A is only illustrative. In either bipolar orunipolar output circuits, the ignition event may be divided into twodischarge events. The first discharge event occurs when a storagecapacitor (not shown) of the exciter 13 or 15 discharges a high energypulse into the output circuit. The second discharge event occurs whenthe inductor L1 stores the energy from the pulse and discharges itthrough the igniter plug and diode D1.

When the igniter plug 1 fails, it typically fails such that the outputcircuit 19 appears as an open circuit with respect to the exciter 13. Inorder to dissipate the high energy pulse from the output of the exciter13 when the igniter plug 1 fails, a resistor is commonly employed toshunt the pulse to ground. Such resistors are often called "safety"resistors. The safety resistors typically each have a value of 1k to 10kOhms. When the igniter plug 1 fires, the impedance of the arc is verysmall (i.e. usually measured in milliohms), approaching a short circuit,and the discharge of the exciter 13 is very rapid because the timeconstant of the output circuit 19 is short. If the igniter plug 1 failsto fire (an arc fails to form due to insufficient ionization), then theimpedance of the output circuit 19 remains very large, approaching anopen circuit, and the discharge of the exciter 13 must seek analternative path that is provided by the safety resistor. Because thesafety resistor has a high resistance value relative to the lowresistance at the gap of the igniter plug 1 when a spark is generated,the rate of discharge of the exciter 13 through the resistor is longcompared to the rate of discharge when a healthy spark has beengenerated.

Referring briefly to the exemplary discharge waveforms of FIGS. 2A and2B, a healthy ignition system 17 discharges the high energy pulse fromthe exciter 13 into the output circuit 19 in approximately three (3)microseconds as indicated by the waveform 31 in FIG. 2A. The normaloutput of the exciter 13 is a narrow high voltage pulse occurring at aregular interval (i.e., 3 microseconds, 2500 volts, once per second).Repeated high energy pulses are typically automatically generated by theexciter 13 in order to provide a series of ignition sparks that protectagainst a flameout of the engine 23 during critical times such aslanding and takeoff of the associated aircraft where a manual initiationof an ignition to restart the engine cannot be safely done.

It will be appreciated by those skilled in ignition systems for gasturbine engines that the waveform 31 is only the initial portion of thefull waveform for an entire discharge event, which is typically 150microseconds long. When the waveform 31 crosses a reference ground forthe exciter 13 and output circuit 19, the exciter has fully dischargedinto the inductor L1 of the output circuit. In FIG. 2A, the waveform 31crosses the reference ground at a time 3.0 microseconds after the highenergy pulse from the exciter 13 is initiated. After the exciter 13 hasfully discharged into the output circuit 19, the waveform 31 thereafterrepresents the signal appearing at the output of the exciter generatedby the discharging of the energy from the output circuit through theigniter plug 1 so as to create a spark. As used hereinafter, the term"discharge event" refers to the discharging of the high energy pulsefrom each of the exciters 13 and 15 into the associated output circuits19 and 21, respectively.

In contrast to the relatively fast discharge event of a high energypulse in a healthy system, a discharge event through the safety resistoroccurs in approximately 5.8 milliseconds as suggested by the exemplarywaveform 33 of FIG. 2B. Because of this virtually three orders ofmagnitude difference in the discharge rates between a healthy dischargeevent and a failed discharge event, the discharge waveform can be usedto monitor the health of the ignition system 17. Although the dramaticdifference in discharge rates between a healthy discharge of the exciter13 and a failed discharge may be used to monitor the performance of theignition system 17, it cannot distinguish between a failure of theigniter plug 1 and a failure of the exciter 13; yet, failure of eithercomponent can cause the system 17 to fail to generate a spark andinstead dissipate the high energy pulse through the safety resistor 27.

In accordance with one important aspect of the invention, the monitoringdevice 11 derives a low voltage signal from the output of each of theexciters 13 and 15 and provides an indication of the state of health foreach of the igniter plugs 1 and 2 as distinguished from the state ofhealth of the exciters 13 and 15 such that a failed discharge event canbe diagnosed as resulting from either a malfunctioning plug that needsreplacing or a malfunctioning exciter that requires servicing. The lowvoltage signals derived from the outputs of the exciters 13 and 15duplicate the waveform at the inputs of associated output circuits 19and 21, respectively. If the low voltage pulses indicate that the pulsesgenerated by the exciter 13 or 15 persistently exceed a predeterminedvoltage value, then there is a high probability that the pulses from theexciters are capable of creating a spark at the igniter plugs 1 and 2.If failed discharge events such as illustrated in FIG. 2B arepersistently detected while either of the exciters 13 or 15 ispersistently providing voltage pulses exceeding the predeterminedvoltage value, the monitoring device 11 will provide an indication thatthe associated plug has failed. On the other hand, if the exciter is notpersistently providing voltage pulses exceeding the predeterminedvoltage value, the monitoring device will indicate the exciter hasfailed, and disallow an indication of a failed igniter plug.

Like the ignition system 17, the monitoring device 11 comprises twochannels, each channel receiving an output from a voltage dividernetwork 27 or 29 connecting the output of each exciter 13 or 15 toground. For purposes of economy, it is desirable to incorporate thesafety resistor into the voltage divider, although it is not necessary.The first channel of the monitoring system 11 is associated with one ofthe two channels of the ignition system 17 and comprises an exciterdetector 35 and an igniter detector 37 connected in parallel so thateach receives the series of low voltage signals from the voltage divider27. Similarly, the second channel of the monitoring system 11 isassociated with a second of the two channels of the ignition system 17and comprises exciter detector 39 and an igniter detector 41 connectedin parallel so that each receives the series of low voltage signals fromthe voltage divider 29. Each of the exciter detectors 35 and 39 monitorsthe series of high voltage pulses delivered to the output circuit 19 or21 by way of the series of low voltage pulses from the voltage divider27 or 29 and detects persistent voltage levels for the high energypulses that are less than a predetermined value representing a nominalminimum voltage generated by exciter 13 when it is healthy. Each of theigniter detectors 37 and 41 also monitors the high voltage pulses by wayof the low voltage pulses from the voltage divider 27 or 29 and detectspersistent discharging of the high energy pulses at a rate much lessthan a predetermined rate representing a nominal minimum rate ofdischarging for the exciters 13 and 15.

When either of the exciter detectors 35 and 39 detects that the highenergy pulses are persistently failing to rise above the predeterminednominal minimum voltage, its output is asserted. The outputs of the twoexciter detectors 35 and 39 provide the inputs for a AND gate 43. Theactive states of the outputs from the exciter detectors 35 and 39 arelow or a logic zero state so that failure of either exciter 13 or 15results in the output of the AND gate 43 assuming a logic zero state.The predetermined nominal minimum voltage used as a reference by each ofthe exciter detectors 35 and 39 corresponds to a minimum voltage thatassures ionization of the spark gap of a healthy output circuit 19 or21.

The AND gate 43 provides its output to the inputs of two NAND gates 45and 47. Together, the AND gate 43 and NAND gates 45 and 47 encode thestate-of-health signals from the exciter detectors 35 and 39 and theigniter detectors 37 and 41 to provide a two-wire output 49 that can beeasily extended as a thin cable for remotely locating a system statusdisplay 51. At the remote location of the display 51, a decoder 53decodes the two-bit signal so that the display may indicate thestate-of-health of each igniter plug 1 and 2 (i.e., "PLUG 1" and "PLUG2" on the display) and the state of exciters 13 and 15 ("SYSTEM OK" or"SYSTEM FAILURE" on the display).

When either of the igniter detectors 37 and 41 detects failure to sparkat one of the igniter plugs 1 and 2, it asserts its output to a state ofan active logic zero (the same as the exciter detectors), which providesa second input to one of the NAND gates 45 or 47. The encoding providedby the three gates 43, 45 and 47 is as follows:

    ______________________________________                                                         Outputs                                                      State of Health    Gate 45  Gate 47                                           ______________________________________                                        "SYSTEM OK"        0        0                                                 "PLUG 1"           1        0                                                 (Igniter plug 1 failed)                                                       "PLUG 2"           0        1                                                 (Igniter plug 2 failed)                                                       "SYSTEM FAILURE"   1        1                                                 ______________________________________                                    

The outputs from the monitoring device 11 cooperates with the decoder 53to provide a diagnostic output that indicates to either maintenancepersonnel, the engine control unit apparatus, or the user of the engine23 (e.g., a pilot) the state-of-health of the ignition system and, mostimportantly, the source of a problem if one exists.

FIG. 3 illustrates an analog implementation of the monitoring device 11in FIG. 1 according to a preferred embodiment. As with the descriptiongiven in connection with FIG. 1, only one of the two channels in themonitoring device of FIG. 3 will be described in detail since the secondchannel of the device is a functional duplication of the first, exceptthe first channel is with reference to a first channel of the ignitionsystem and the second channel is with reference to a second channel ofthe ignition system.

When the ignition system 17 (FIG. 1) is initially connected to a DCpower source Vin, it immediately begins producing sparks. At the sametime power is applied to the ignition system 17, it is also applied tothe monitoring device in FIG. 3 via a power supply filter 61. A smallcurrent flows from the DC power source Vin through diode D4, whichprevents damage if reverse polarity power is inadvertently applied. Thecurrent charges capacitor C7, which provides smooth and noise free powerto the logic. The DC voltage biases integrated circuits U1 through U8,establishes the reference voltages REF1, REF2, and REF3, and establishesan initial condition (charge) on capacitor divider networks C1, C5 andC2, C6.

Turning first to the detection of a failed exciter, the diagnostic logicmonitors the high voltage discharge at the output of the exciter 13where it enters the output circuit 19 going to the igniter plug 1.Safety resistor R30 and resistor R31 form the voltage divider network 27shown in FIG. 1. The signal at the node between the two resistors R30and R31 is an attenuated duplicate of the output pulse from the exciter13, and it is further attenuated by the voltage divider consisting ofR17, R18 before being applied to the monitoring device at the input of avoltage comparator U1.

The output of comparator U1 will transition from a low to a high levelwhenever the voltage at its noninverting (+) input exceeds the voltageat its inverting (-) input, which is connected to a threshold referencevoltage REF1. A small current flowing through resistor R19 into zenerdiode D3 provides a stable voltage for the reference voltage REF1 (i.e.,6.2 volts). The ratio of resistors R17, R18 is adjusted so that the (+)input of comparator U1 equals the reference voltage REF1 when the outputof the exciter 13 reaches a fixed percentage of its expected output(e.g., 2000 volts as indicated in the exemplary waveforms of FIGS. 2Aand 2B, which is 80% of a pulse normal 2500 volts). Thus, each time theexciter 13 provides a pulse to the output circuit 19 that is at least2000 volts, a pulse is created at the output of comparator U1.

Resistor R3 is a pull-up resistor that is only effective when the pulsefrom the exciter 13 is sensed across resistor R31, but that is the onlytime that the output of comparator U1 could be high. Furthermore, theamplitude of the pulse across the monitoring resistor R31 isconsistently approximately 12 vdc, whereas if the pull-up resistor R3was returned to Vin which varies from about 10 vdc to 30 vdc, thenadditional circuitry would be required to protect the gate of the MOSFETQ3.

In order to detect failure of the exciter 13, the output pulses from thecomparator U1 are applied to the input of a persistence detectorconsisting of MOSFET Q3, an integrator circuit composed of resistor R7and capacitor C3, and a comparator U3. Proper operation of the exciter13 periodically causes a pulse that turns on MOSFET Q3, which in turnresets the integrator circuit. MOSFET Q3 is a fast switching device sothat even very short pulses derived from the waveform of a healthyignition will be sufficient to discharge capacitor C3 by turning onMOSFET Q3. Thus capacitor C3 will not charge to a very high level beforebeing reset by the next pulse when the waveform is consistently one of ahealthy ignition event.

When a malfunction occurs and the exciter 13 ceases to deliver outputpulses or delivers substandard output that is recognized by the pulsevoltage discriminator, then MOSFET Q3 is no longer triggered and thecapacitor C3 of the integrator circuit begins to accumulate a charge.Eventually, the voltage on the capacitor C3 will equal the thresholdreference voltage REF3 set by voltage divider network of resistors R11and R12, and the output of comparator U3 will be forced low, indicatinga failure of the exciter 13. Resistor R23 is a pull-up resistor thatnormally keeps the output of comparator U3 high and maintains a reversebias on latching diode D1. If sufficient time passes between pulses tothe base of MOSFET Q3, the output of comparator U3 switches low, diodeD1 becomes forward biased and the gate of MOSFET Q3 is pinned low sothat the persistence detector cannot after which be reset. Thus, theoutput of the comparator U3 will be latched in the low state, indicatingfailure of the exciter 13. If the diode D1 is omitted, then the circuitwill recover upon resumption of standard output pulses by the exciter13, and the failure indication will be removed. In some applications,resetting the failure indication upon resumption of healthy outputpulses may be a preferred operating mode, so the use of diode D1 may beconsidered optional.

As illustrated in FIG. 3, the monitoring device thus far described forthe exciter 13 is duplicated for the exciter 15. In this regard, theexciter detector for monitoring exciter 15 comprises voltage dividernetwork R20, R24, comparator U2, and pull-up resistor R4, whereas thepersistence detector comprises MOSFET Q4, integrator circuit R8, C4,comparator U2 and diode D2. Inasmuch as the exciter detector andpersistence detector for the exciter 15 function in the same manner asthe detectors for the exciter 13, a description of that function willnot be repeated herein.

Although it is not fundamental to this invention, the two outputs ofcomparators U3 and U4 indicating left and right channel exciter failureare shown tied together in the preferred embodiment in FIG. 3. Theresulting signal is a wired AND logic function--i.e., either channelgoing low to indicate a failure causes the combined output to go low.The definition of this signal is modified as the failure of either orboth exciters 13 and 15.

Turning now to an explanation of the circuitry for detecting failure ofthe igniter plugs 1 and 2, and referring again to the left channel inFIG. 3, the monitor point at the junction of safety resistor R30 andresistor R31 is also connected to the input of an igniter health monitorcircuit. Each pulse biases transistor Q1 through resistor R1 and causesit to turn on when the voltage exceeds the base emitter voltage oftransistor Q1 (e.g., approximately 0.7 volts corresponding to 148 voltsat the output of the exciter).

Before examining the effect of transistor Q1, it is necessary to definethe initial condition of the circuit as represented by the voltage atthe node of capacitive voltage divider comprising capacitors C5, C1.When DC power is first applied to the diagnostic circuitry, the twocapacitors C5, C1 charge essentially instantaneously in inverseproportion to their values that are chosen so that the common node willhave an initial voltage of about 75% of the supply voltage. It will beappreciated by those skilled in the art of circuit design that, asidefrom the acquisition of the initial value, the effect of capacitors C1,C5 is of a simple integrator, and they behave essentially the same as ifa single capacitor with a value equal to the parallel combination ofcapacitors C1 and C5 were located in the position of capacitor C1.

The integrator formed by the capacitors C1, C5 is charged by a currentoriginating at the DC power supply and flowing through the seriesconnected resistors R15, R13 provided that the output of comparator U5is high (i.e., off, since in the illustrated implementation of theinvention the outputs of the comparators are all open-collectortransistors with their emitters tied to ground). Furthermore, theintegrator C1, C5 is discharged by shunting a current to ground viaresistor R5 when transistor Q1 is turned on. In contrast to the"resetting" of the integrator R7, C3 in the exciter health monitor byMOSFET Q3, in the case of integrator C1, C5 the discharge is neitherinstantaneous nor complete but rather has a rate and duration set by thevalue of resistor R5 and the on-time of transistor Q1, respectively.Thus, very narrow pulses occurring at the output of the exciter 13 willcause negligible effect on the state of charge of the integrator C1, C5because of the corresponding short on-time of transistor Q1.

During the interval between pulses (when transistor Q1 is off) theintegrator C1, C5 will continue to charge toward the DC supply voltageVin due to resistors R15, R13. The comparator U5 compares the value fromthe integrator C1, C5 with a threshold reference voltage REF2, which isa voltage created by a voltage divider comprising resistors R9 and R10and which typically will have a value of about 20% of the DC supplyvoltage Vin.

If the igniter plug 1 fails to fire, the high voltage pulse from a tankcapacitor (not shown) of the exciter 13 will seek an alternate dischargepath through safety resistor R30 and resistor R31 to ground. The rate ofdischarge through these resistors is several orders of magnitude slowerthan if the pulse had discharged through the igniter circuit. Theon-time of transistor Q1 will be long, thus significantly dischargingthe integrator C1, C5 so that its value approaches the thresholdestablished by reference voltage REF2. After several consecutive misses(pulses where the igniter plug 1 fails to fire) the voltage at theintegrator C1, C5 value will reach the reference voltage REF2 and willcause the output of comparator U5 to transition to a low stateindicating that the igniter plug 1 has failed persistently. Althoughlatching this condition is not fundamental to the invention, theimplementation of FIG. 3 performs a latching function in that once thefailure causes the output of comparator U5 to go low, the pull-up effectof resistors R15, R13 is reversed and becomes a pull-down (discharge)effect via resistor R13 to the output of comparator U5 which is now atground potential. Subsequently, the voltage value at the integrator C1,C5 can only drift further toward ground, thereby ensuring that itsvoltage will stay below the reference voltage REF2 until DC power isremoved.

The persistence detection in the igniter detector is important becausethere is typically a large variation in the spark-to-spark performanceof an ignition system. The variation is caused by minor deviations inthe output of the exciter, the age of the plug, and large deviations inperformance of the igniter plug due to varying conditions in thecombustion chamber of the turbine that effect ionization. An occasionalmiss in a sequence of normal sparking is not sufficient to judge afailed igniter. The integrator is set to partially recover (charge) inthe normal interval between sparks; its charge must be less than thedischarge caused by the miss, but enough so that only persistent (e.g.,8 out of 10) consecutive failures will discharge the exciter to theigniter failure threshold.

As with the components of the exciter detector 35 in FIG. 1, it will beseen in FIG. 3 that the components of the igniter detector 37 areduplicated for the right channel; with components resistors R2, R6, R14,R16, transistor Q2, capacitors C6, C2, and comparator U6 functionallyreplacing resistors R1, R5, R13, R15, transistor Q1, capacitor C5, C1and comparator U5, respectively. The output of comparator U6 will thusindicate failure of the right igniter plug 2 in the same way that theoutput of comparator U5 indicated failure of the left igniter plug 1.

Fundamental to the operation of any diagnostic circuitry is theprevention of any false positive outputs (reporting a failure when thereis not). In the preferred embodiment of FIG. 3, a false positive couldoccur if the igniter detector 37 reported a failed igniter when theactual cause of failure to spark was a degraded exciter output. Such acondition occurs when the output pulses from the exciter occur regularlybut their peak voltages are below the threshold necessary to ionize theigniter gap to create a spark. When such a condition exists, the outputof the igniter detector 37 would be invalid.

In keeping with the invention, in order to prevent false positivereports from the igniter detector 37, the preferred embodiment relies ondetecting the degraded output of the exciter 13 sooner than the earliestpossible detection of a failure of the igniter plug 1 and using theformer to preclude reporting the latter. Although there are manypossible circuit implementations that could accomplish this prioritizeddetection, the embodiment of FIG. 3 achieves priority control by settingthe relative persistence requirements of the exciter detector 35 andigniter detector 37 so that the exciter detector will react faster(i.e., exciter failure that must persist for five (5) seconds versusigniter failure that must persist for ten (10) seconds).

The remaining block in FIG. 3 is the output circuit 63 consisting ofoutput encoding logic and output line buffers. Inputs to the encodinglogic are the four individual failure signals LEFT EXCITER, LEFTIGNITER, RIGHT EXCITER and RIGHT IGNITER. In some applications all ofthese signals may be useful, or conversely they may be combined into asfew as one output indicating FAILURE but offering no additionalinformation as to which one caused the report. The preferred embodimentcombines these signals into two output bits, either of which can be ON(pulled low) or OFF (open circuit), allowing the system to report fourpossible states:

    ______________________________________                                        SYSTEM NORMAL        ON/ON                                                    LEFT IGNITER FAILURE OFF/ON                                                   RIGHT IGNITER FAILURE                                                                              ON/OFF                                                   SYSTEM FAILURE       OFF/OFF                                                                       (EITHER OR BOTH                                                               CHANNELS                                                 ______________________________________                                    

The output of comparator U5 is normally high and when applied to the (-)input of comparator/buffer U7 will force its output low (ON). Uponfailure of the left igniter the output of comparator U5 will go lowforcing the output of comparator U7 high (OFF). Similarly, the output ofcomparator U6 can force the output of comparator U8 high (OFF) to reportthe failure of the right igniter.

Additionally, the comparators U7 and U8 are of a type which haveopen-collector outputs which are OFF (i.e. high, 1) when their powersupply is interrupted; thus reporting a "SYSTEM FAILURE" because neitherchannel is capable of generating a spark, and the diagnostic circuitryhas also lost its operating power.

The two exciter detectors of FIG. 3 have their outputs connectedtogether at a node which also includes the cathodes of diodes D5 and D6.This arrangement performs a logical AND operation like gate 43 in FIG. 1such that the node will be high if and only if the outputs ofcomparators U3 AND U4 are high (both exciter channels are operating).Either exciter detector output going low (failure indication) causes thenode to go low which forward biases diodes D5 and D6 to pull the inputsof both comparators U7 and U8 low thus forcing their outputs to OFF/OFFand reporting an exciter failure. Once the SYSTEM (exciter) FAILUREoccurs and the BIT1 and BIT2 outputs are in the OFF/OFF state,subsequent failure of either igniter plug 1 or 2 will not cause anychange (other than to force an already OFF output to remain OFF). Thusreporting the SYSTEM FAILURE will also preclude reporting an igniterfailure and the integrity of the diagnostic system is protected from afalse positive. The choice of OFF/OFF for SYSTEM FAILURE also guaranteesthat loss of power to the exciter (and/or power to the diagnosticmonitors) will cause a SYSTEM FAILURE indication.

The diagnostic outputs BIT1 and BIT2 normally exit the ignition systemvia a connector and are connected via a wiring harness to an informationdisplay unit or to the engine control unit (ECU). The final components(resistor R21, diode D7 and resistor R22, diode D8) are current limitingresistors and voltage clamping zener diodes that protect the diagnosticcircuit from electrical transients which might enter the ignition systemthrough these output lines.

Turning now to an alternative embodiment of the invention illustrated inFIG. 4, several performance improvements can be achieved by a digitalcircuit implementation of the decision logic. The interface to the highvoltage output of the ignition system uses the same voltage divider 27as was already discussed with reference to FIGS. 1 and 3. The signal isthen applied to the inputs of two level detectors 71 and 73. The firstlevel detector 71 compares the exciter signal to a reference voltageREF1 that represents the minimum acceptable level which will be acceptedas a valid exciter output. The output of detector 71 is a digital signalthat will be low during the time that the exciter output is above REF1(i.e., 2000 volts). The second level detector 73 compares the excitersignal to a reference voltage REF2 which is set at a low threshold sothat the exciter signal will exceed it for virtually the entiredischarge event. Therefore, the output of detector 73 is a digitalsignal which will be low during the time that the exciter output isabove REF2 (i.e., 500 volts). To illustrate the significance of thesetwo digital signals, they can be related to the energy of the tankcapacitor of the exciter. Energy in a capacitor is:

    E(Joules)=1/2C(Farads)*V.sup.2 (volts.sup.2)

At the beginning of the discharge event, both signals go low; at thistime 100% of the energy remains in the tank capacitor. If REF1 is set at80% of the nominal exciter output voltage (e.g., 2000 of 2500 volts)then the first signal from detector 71 will go high at the time when thetank capacitor has discharged to 2000 volts, and the remaining energywill be 64% of the initial energy, with 36% discharged during the timethe first signal was low. The second signal from detector 73 will gohigh when the tank capacitor has discharged to reference voltage REF2which is 20% of the initial voltage, and at that time only 4% of theinitial energy remains in the capacitor. At the time the second signalgoes high 96% of the energy has been discharged. Additionally, betweenthe time the first signal went high and the time that the second signalwent high 60% of the energy is discharged. In this sense, the rate ofdischarge is related to passing through two different voltage (charge)levels in a measured period of time.

In keeping with the invention, there is sufficient information in theindividual and relative timing and duration of these two digital signalsto diagnose the health of both the exciter and its igniter plug. Theamount of diagnostic information that can be usefully extracted dependson the complexity of the decision making logic that processes the twosignals.

The broadest implementation of this logic utilizes a microprocessor (notshown) that uses the signals from detectors 71 and 73 as two of itsinputs. It may also incorporate additional inputs from other parts ofthe exciter that would allow a more detailed diagnosis to be performed.For example, the spark duration might be sensed via the current throughinductor L1 (FIG. 1), or a thermocouple could detect overheating ofcircuit components, and provide the microprocessor with a warning ofimpending failure.

The microprocessor executes a program that implements decisionalgorithms in accordance with this invention in order to determine thestate of health of both the exciter and its associated igniter plug. Itcan also control communication of the results to minimize the complexityof interconnection by using well known techniques like serial datatransfer.

As an alternative to a microprocessor implementation of the diagnosis,the same results can be achieved with discrete digital logic (i.e.,gates, counters, latches, etc.) as shown by the embodiment in FIG. 4.This circuit implements a single channel of a digital igniter detectorin keeping with the invention. The primary improvement over the igniterdetector of FIG. 3 is that the evaluation of whether the igniter plugfired or missed is done for each individual spark attempt (i.e., exciteroutput pulse). When DC power is first applied to the circuit of FIG. 4,a power-up reset circuit 75 generates a pulse to initialize the circuitthat sets a flip-flop 77 so that its "Q" output is high, reporting agood igniter (in this case the first diagnosis of the health of theigniter plug has not been performed so it is presumed to be good). Whenthe first discharge occurs there will be a pulse at the output of leveldetector 71 if, and only if, the pulse exceeds 2000 volts; thus if theexciter output is degraded and will not necessarily fire the igniterplug, no pulse will occur and no decision making process will beinitiated. When a valid output occurs, the pulse from detector 71triggers a one-shot timer 79, which produces a single pulse with a fixedduration (e.g., 1 millisecond). The trailing edge of this pulse clocksflip-flop 77, which samples its "D" input and latches that value intoits "Q" output.

If a spark occurs, then the discharge will be completed much sooner than1 millisecond, and the output of the second level detector 73 will behigh at the end of the 1 ms delay. This signal, applied to the "D" inputof flip-flop 77, causes the output of the flip-flop to be high (nochange since it already is high), reporting a good spark.

If no spark occurs, then the discharge will be much longer than 1millisecond because its rate depends on the safety resistor (R31 in FIG.3). In this case, when the output of level detector 73 is sampled byflip-flop 77 after the delay created by one-shot 79 it will still be ina low state indicating that the discharge is still in progress. Theoutput of flip-flop 77 will thus transition to a low state reporting amissed spark.

During normal operations, a NOR gate 81 blocks pulses from one-shot 79because its other input from flip-flop 77 is high. After the firstmissed spark, the low output of flip-flop 77 allows the pulses throughgate 81 and to the input of the missed-sparks counter 83. The counteroutput will have a binary value of zero because it was held reset by theoutput of flip-flop 77 prior to the first miss. If a second consecutivemiss occurs then the counter 83 will increment to one. A thirdconsecutive miss will increment it to two, and so on.

When the counter 83 reaches a count of eight, after the ninth miss, itsQ8 output goes high which reports a failed igniter plug, having detectednine consecutive misses. Other implementations of a digital embodimentcould require any integer number of misses as the definition of a failedigniter. The Q8 output sets latch 85, the output of which reports"FAILED PLUG"; this indication will remain until the next powerinterruption cycle.

If the failure of the igniter plug was only an intermittent conditionand it recovers and a spark occurs at any time before the counterreaches eight, then the output of flip-flop 77 will return to the highstate and will reset the counter to zero. No report will be made of afailed igniter because the nine-in-a-row requirement was not satisfied.

The intermittent sparking condition is still useful information,however, because it usually indicates that the plug may be nearing theend of its useful life and if the output of flip-flop 77 is monitoredvia another latch 87 then an additional diagnostic signal is availableto report an "INTERMITTENT PLUG". A persistence counter could similarlybe employed to only report "INTERMITTENT PLUG" if a certain ratio ofintermittent misses was exceeded (e.g. any 5 misses out of a sequence of10 sparks).

In a manner similar to that illustrated in FIG. 1, the outputs oflatches 85 and 87 may be encoded by an encoder 89 of conventional designfor communicating the diagnostic signals to a display 91 via a serial orparallel communications cable 93. At the end of the cable 93, thedisplay 91 decodes the diagnostic signals in a decoder 95 and provides auser with an indication of the state of health of the system in keepingwith the invention at indicators 97.

As indicated in FIG. 4, the diagnostic signals from additional ignitionchannels can be encoded in the encoder 89 for communicating to thedisplay 91 via the cable 93. As illustrated in FIG. 1, a second channelmay be incorporated into the ignition system. An ignition detector 95for a second channel that is functionally identical to the illustratedigniter detector for the first channel provides "FAILED PLUG" and"INTERMITTENT PLUG" diagnostic signals to the encoder 89. Diagnosticsignals from the exciter detectors for both channels are also providedas inputs to the encoder 89. Each of the exciter detectors may be adigital version of the analog-type exciter detectors illustrated in FIG.3 or they may be the analog-type devices themselves. Finally, otherdiagnostic devices 97 associated with the ignition system may also beencoded and delivered to the display 91 via the cable 93. For example, athermocouple may be one of the devices 97, which could be attached to acomponent of the ignition system to indicate an overheating conditionindicative of the system's impending failure.

These methods can be extended by additional logic, or additionalprogramming if a microprocessor is employed, to generate the exciterhealth outputs using the same signals from the level detectors.

I claim:
 1. An apparatus for diagnosing the health of an ignition systemfor a turbine engine, where the ignition system includes an exciterproviding high energy pulses to an igniter plug for igniting the fuel ofthe turbine engine, the apparatus comprising:an exciter detector fordetecting voltage levels for the high energy pulses that are less than apredetermined value representing a nominal minimum voltage generated bythe exciter when it is healthy; an igniter plug detector for detectingwhether the high energy pulses produce sparks at the igniter plug; and adiagnostic circuit responsive to the exciter and igniter plug detectorsfor reporting a failure of the igniter plug only when the detectorsindicate the exciter is healthy and the sparks are not being produced atthe igniter plug, the diagnostic circuit including an output forreporting the state of the health of the ignition system.
 2. Theapparatus of claim 1 wherein the exciter and igniter plug detectors andthe diagnostic circuit are incorporated into transportable automatictest equipment.
 3. The apparatus of claim 1 wherein the exciter detectorand the diagnostic circuit are incorporated into the ignition system. 4.The apparatus of claim 1 wherein the igniter plug detector detectspersistent discharging of the high energy pulses at a rate much lessthan a predetermined rate representing a nominal minimum rate ofdischarge for the igniter plug when it is healthy and the diagnosticcircuit includes an output for reporting the state of health of theignition system.